Protective coatings and process for producing the same



June 1, 1965 a. D. oxx, JR 3,186,070

PROTECTIVE COATINGS AND PROCESS FOR PRODUCING THE SAME Filed July 3,1961 Weight Gain (mg /cm Time Hours Fig. 2. lnvenfor Gordon 0. Oxx, Jr.

His A horney- United States Patent Office 3,185,070 Patented June 1,1965 3,186,070 PROTECTIVE COATINGS AND PROCESS FOR PRODUCING THE SAMEGordon D. Oxx, In, Shaker Heights, Ohio, assignor to General ElectricCompany, a corporation of New York Filed July 3, 1961, Set. No. 121,41911 Claims. ((11. 29-194) This invention relates to bodies formed ofcolumbium and columbium-base alloys and more particularly to protectivecoatings for such bodies and to processes for protective coatings.

Rapid advances in the technological field dealing with apparatus, suchas gas turbines and the like, have resulted in greatly expanded need forrefractory metals and materials capable of retaining high strength atelevated temperatures in environments which are often destructive. Inthe case of gas turbines, many of the component parts must be capable ofwithstanding severely oxidizing conditions for reasonable lengths oftime. Columbium and alloys of columbium have great potential forstructural applications at elevated temperatures, not only because oftheir high strength-to-weight ratio, but also because of theirrelatively low rates of oxidation as compared with other fabricablerefractory metals and alloys. However, even though the oxidation ratesof columbium and columbium-base alloys are comparatively low, they stilloxidize at rates two to three times greater than acceptable. For thisreason, protection of the exposed surfaces of these alloys is requiredfor practical applications.

Attempts have been made in the past to protect columbium andcolumbium-base alloys from oxidizing environments by enclosing them inprotective containers or jackets or by reacting the surface of the metalwith another substance to form a protective compound. Various metalshave been reacted with the exposed surfaces of columbium andcolumbium-base alloys, and included among this group is aluminum andaluminum alloys reportedly containing small additions of materials suchas beryllium, cerium, chromium, copper, cobalt, magnesium, molybdenum,silicon, titanium, vanadium, and zinc. However, adequate long-time (over100 hours) protection of the metals with which this invention isconcerned has not been obtained since the coatings formed by reactionwith aluminum have, for the most part, been brittle and subject tocracking upon thermal cycling.

A principal object of this invention is to provide columbium orcolumbium-base alloy bodies having protective coatings rendering themsuitable for use at elevated temperatures in oxidizing environments.

Another object of this invention is to provide columbium orcolumbium-base alloy bodies having protective coatings consistingprincipally of the columbium-aluminum compounds, Cb Al or Cb Al.

An additional object of this invention is to provide a protectivecoating for columbium or columbium-base alloy bodies, which coatingconsists principally of the compounds CbgAl and/ or Cb Al .andcontaining further additions which improve the oxidation resistance ofthe compounds.

A further object of this invention is to provide a novel process forproducing a protective coating on the exposed surfaces of columbium orcolumbium-base alloy bodies, rendering these bodies suitable for use atelevated temperatures in oxidizing environments.

A still further object of this invention is to provide a process forproducing a columbium aluminide protective coating on the exposedsurfaces of columbium or columbium-base alloyed bodies.

Additional objects and advantages of this invention will be in partobvious and in part explained by reference to the accompanyingspecification and drawings.

In the drawings:

FIG. 1 is a graph showing the oxidation rate of selected coatingcompositions as a function of time; and

FIG. 2 is an enlarged cross section of a coating of this inventionshowing the reaction products.

Generally, a body according to this invention comprises a columbium orcolumbium alloy base which has a protective layer formed integrally onan exposed surface thereof. The protective layer or coating is composedof one of the compounds Cb Al or CbgAl or some combination of these twocompounds and may contain additional alloying ingredients such aschromium, silicon, or titanium dissolved in the compound to furtherimprove the oxidation resistance.

The process of the invention, which is important in obtaining a coatinghaving the required protective properties, generally comprises preparingan alloy of aluminum with one of the metals, silver, tin or lead, thealuminum normally being present in amounts of from about 17 to 23 weightpercent, immersing the columbium or columbiumbase alloyed body in themolten bath and retaining it in the bath for a period of time sufficientto react the aluminum therewith. When a coating of proper thickness hasbeen obtained, the body is removed from the bath. If desired, thealuminum-silver, tin or lead bath may contain appropriate amounts ofchromium, silicon, or titanium or alloys thereof which will be alloyedwith the columbium aluminide formed on the surface of the base article.

Suitable materials which may be alloyed with columbium comprisetungsten, molybdenum, tantalum, zirconium, and titanium, this list notbeing complete but given only for purposes of illustrating suitablecolumbium-base alloys. It is, of course, desirable that the percentageof alloying ingredient added to the columbium not exceed an amount whichwould preclude the proper formation of a protective coating on the body.

Columbium containing amounts of tungsten up to 15 weight percent;molybdenum, 5 weight percent; tantalum, 33 weight percent; titanium, 8weight percent; and minor amounts of zirconium have been found to besuitable for high temperature use.

Three compounds are formed by the reaction of aluminum with columbium.These compounds are CbAl which has a melting point in the vicinity of1620 Q; Cb Al, with a melting point of 1700 C.; and Cb Al, which has amelting point on the order of 2260 C. There are eutectics at 656 C.,1520" C. and 1640" C. between A1 and CbAl CbAl and Cb Al, and CbzAl andCb Al, respectively. On the surface of an unalloyed columbium sampledipped in aluminum of activity essentially equal to one, all three ofthese phases will form, and on alloys of columbium, it is probable thateven more compounds will be present. By reacting columbium with alumiumonly, thus causing an activity of one, at a temperature below 1620 C.,it is thermodynamically impossible to avoid formation of CbAl as theouter layer. It has been found that the presence of CbAl is extremelyharmful to the coating, this compound resulting in the formation ofcracks and otherwise having relatively poor corrosion resistance. It hasnow unexpectedly been found that by lowering the activity of thereacting aluminum, it is possible to avoid formation of CbAl and form aprotective coating consisting only of Cb Al, Cb Al, or some combinationof these compounds, plus possibly some additional complex compoundswhere modifying alloying agents have been included.

By combining the aluminum with a metal from the group consisting ofsilver, tin and lead, these metals being substantially inert withrespect to columbium and columbium-base alloys, it is possible to lowerthe activity of the aluminum and obtain only the compounds Cb Al andCbgAl. Thus, when columbium or columbium-base bodies are dipped into amolten bath containing alumium, in percentages not exceeding about 23weight percent, the compounds Cb Al and Cb Al are formed as an integralcoating on the exposed surface of the base metal. It is also possible ifthe aluminum content is within the lower end of the range that thecoating will consist essentially of the high temperature compound Cb3A1.

The general process used in forming a protective coating on the exposedsurfaces of columbium and columbium-base alloy bodies is as follows. Asuitable base is prepared and immersed in a bath containing from about17 to 23 weight percent aluminum, balance substantially all a metalwhich will dissolve the aluminum and yet be non-reactive with the basematerial. It has been found that silver, tin and lead are extremely wellsuited for this purpose since aluminum is soluble in each and since noneof these materials is reactive with columbium. Immersion of the basepiece within the molten bath is maintained only for a time sufi'icientto develop a coating of desired thickness. Generally, the temperaturesof the molten baths are in the vicinity of 1150 C. to 1250 C., althoughthe particular temperature selected is not especially critical. Ofcourse, it will be understood that the higher temperatures make possiblethe use of shorter immersion times, while lower temperatures necessitateslightly longer immersion times. After the article has been immersed inthe molten bath for the required time, it is removed and is then readyfor use.

Illustrating the properties of columbium aluminide protective coatingsdeveloped according to the present invention, unalloyed columbium testspecimens were dipped into a molten silver-aluminum alloy at 1200 C. andallowed to react to form a coating. The molten alloys used contained 10,20, 25 and 35 percent aluminum, balance substantially all silver plus 5percent silicon.

The samples were then tested by holding in flowing air at 2372 F. andperiodically recording weight change. The oxidation rates of therespective coatings are indicated in FIG. 1, where weight gain per unitarea for each sample is plotted against time. While the Ai-5 Si-85 Agsample, curve 10, showed the best life (69 hours), the Al-5 Si-75 Agsample, curve 11, was significantly better with regard to oxidationrate. The 20 hours difference in life of these two samples is probably aconsequence of differences in coating efficiency and thickness, inthermal cycling during test and in conditions of test. The rateditference is of technical significance, however. Under properconditions, the coating that is a consequence of reaction of the 20 Al-5Si-75 Ag bath with columbium has the best oxidation resistance andpotentially the longest life. Curve 12 indicates that high aluminumcontent, composition Al5 Si- Ag, results in a coating having pooroxidation rate and low life.

Similar comparisons were made between samples reacted with 20 Al-80 Agand 25 Al- Ag baths. The rates were higher than in the case of therespective baths with the silicon additions but the relative positionswere the same. In this case, the 20 Al- Ag bath gave a life of 125 hoursat 1300" F. and a rate of about one-fifth that of the 25 Al-75 Ag bath.The latter showed a life of 53 hours.

As a consequence of the above, it is concluded that the 20 Al bathclearly represents the optimum composition with regard to oxidationresistance.

Another important requirement for columbium coatings is that in additionto preventing oxidation, they must prevent penetration of oxygen ornitrogen into the columbium metal. Contamination of the metal by thesegases causes embrittlement. The effectiveness of the 20 percentaluminum-silver alloy coating in preventing this was checked byattempting to bend the specimen at room temperature. The sample bentmore than without cracking.

Additional specimens of unalloyed columbium were dipped into moltenbaths containing alloying agents such as chromium, silicon and titaniumin addition to the aluminum and silver. The properties of thesecoatings, together with the temperature at which they were tested inflowing air are shown in the following table:

Table 1 Composition Temp.

of Life test (hrs.) Al Cr Si Ti Ag C) 20 O 2 0 BaL 1, 260 G9 20 5 0 0Bal l, 260 205 20 5 2 0 Bal 1, 260 393 20 5 0 5 Bal 1, 260 476 20 0 0 0Bal- 1, 300 125 25 0 0 0 Bal- 1, 300 53 10 0 5 0 Del 1, 300 62 20 0 5 0Bal.-. 1, 300 49 2O 5 0 5 Bal 1, 370 5 20 5 0 10 Bal--- 1,370 1 25 0 0 0BaL 1, 370 36 20 5 2 0 Bal 1, 370 51 20 5 5 0 B :11 1, 370 20 5 0 0 Ball1, 370 44 The data set forth in the preceding table clearly indicatesthat additions in the bath of about 5 percent chromium have the mostpotent effect on increasing the life of the specimen. Bath compositionson the order of 20 percent aluminum, 5 percent chromium plus 5 percenttitanium give the best protection at 1260" C. The 20 percent aluminum, 5percent chromium and 2 percent silicon is particularly effective inincreasing the life of the specimen at 1370 C. In all of the samplesmeasured, the coatings consisted principally of the two compounds Cb Aland Cb3A1. Although it is possible that some minor percentage of CbAlwas present, it was felt to be insignificant in its effect upon theprotective coating. After holding in air for hours at 1300 C., thesample dipped in the 20 Al-80 Ag bath showed a microstructure as in FIG.2. The bottom of this figure, area 15, is the columbium substrate. Thecoating appears as two layersa thin layer 16 next to the columbium(probably CbgAl) and heavy, continuous outer layer 17 that is probablyCb Al. X-ray diffraction analyses on this sample indicated the presenceof Cb, Cb Al and Cb Al, but no CbAl Although this invention has beendescribed in connection with specific examples, these examples are meantto be illustrative of rather than limitations on the broad scope of theinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An article of manufacture comprising, a body formed of a metal fromthe group consisting of columbium and columbium-base alloys, and aprotective coating consisting essentially of the compounds Cb Al and CbAl integral with an exposed surface of said body to protect it againstoxidation.

2. An article of manufacture comprising, a body formed of a metal fromthe group consisting of columbium and columbium-base alloys, and aprotective coating consisting essentially of the compounl Cb Al integralwith an exposed surface of said body to protect it against oxidation.

3. An article of manufacture comprising, a body formed of a metal fromthe group consisting of columbium and columbium-base alloys, and aprotective coating consisting essentially of the compound Cb Al integralwith an exposed surface of said body to protect it against oxidation.

4. An article of manufacture comprising, a body formed of a metal fromthe group consisting of columbium and columbium-base alloys, and aprotective coating consisting essentially of a compound selected fromthe group consisting of Cb Al and Cb Al, and up to 15 weight percent ofa metal selected from the group consisting of chromium, silicon,titanium, and combinations thereof dissolved in said selected compound,said protective coating being integral with an exposed surface of saidbody to protect it against oxidation.

5. An article as defined in claim 4 wherein up to 5 weight percentchromium is dissolved in said selected compound.

6. An article as defined in claim 4 wherein up to weight percenttitanium is dissolved in said selected compound.

7. An article as defined in claim 4 wherein from 1 to 5 weight percentchromium and 1 to 5 weight percent titanium are dissolved in saidselected compound.

8. An article as defined in claim 7 wherein 5 weight percent chromiumand 5 weight percent titanium are dissolved in said selected compound. 1

9. An article as defined in claim 4 wherein from 1 to 5 weight percentchromium and from 1 to 5 Weight percent silicon are dissolved in saidselected compound.

10. An article as definedin claim 9 wherein 5 weight percent chromiumand 5 weight percent silicon are dissolved in said selected compound.

11. An article of manufacture comprising, a body formed of a metal fromthe group consisting of columbium and columbium-base alloys, and aprotective coating con- 6 sisting essentially of a compound selectedfrom the group consisting of Cb2A1 and Cb Al, and from 2 to 5 weightpercent silicon dissolved in said selected compound, said protectivecoating being integral with an exposed surface of said body to protectit against oxidation.

References Cited by the Examiner UNITED STATES PATENTS 2,770,031 11/56Schluchter 29-197 2,774,686 12/56 Hodge 117-114 2,947,069 8/60 Carlsonet al. \29197 2,991,197 7/61 Sandoz et a1 117-131 2,992,135 7/61 Finlay117-114 3,055,088 9/62 Cox 1 29-194 3,057,048 10/62 Harakis 29197 X3,078,554 2/63 Carlson 29-497 OTHER REFERENCES Zinc Coatings forProtection of Columbium from Oxidationat Elevated Temperatures, Klopp eta1. (Order PB 161 238 from OTS, US. Dept. of Commerce, Washington 25,DC).

DAVID L. RECK, Primary Examiner.

RICHARD D. NEVIUS, HYLAND BIZOT, Examiners.

1. AN ARTICLE OF MANUFACTURE COMPRISING, A BODY FORMED OF A METAL FROMTHE GROUP CONSISTING OF COLUMBIUM AND COLUMBIUM-BASE ALLOYS, AND APROTECTIVE COATING CONSISTING ESSENTIALLY OF THE COMPOUNDS CB2AL ANDCB2AL